I'm trying to figure out the typical advantages in marginal low light or motion situations to decide if it's worth the upgrade to me. Is it the difference between getting certain shot in focus vs not getting it in focus on a Nikon D5100 because the D5100 was focus hunting, or lost track of the subject, or the subject tracking was just running a bit behind the subject (the latter happens on my compact all the time), for example? Just trying to figure out what it means in concrete photography terms to go with pro full frame equipment.

Ted

The difference lies with the ability of that camera to produce an image in relatively low light that is usable. Noise is the issue. Both the 5100 and the 800 are capable of the same range of iso's which is 100 to 6400, with a boost up to 25000+. The larger sensor of the 800, and others like it, handles the higher iso's with less noise. I am not a techie, but I believe the larger pixels of a full frame has something to do with that. Enthusiast and pro cameras have faster focusing capabilities . Most cameras, amateur or professional, have the ability to focus in relatively low light. Some faster than others.

Another issue which I did not mention in my previous post, is the capability of camera body to handle prolonged usage. How will the body handle some abuse and constant usage? Now there are many cropped sensor cameras that would meet that criteria, at least 5 that I would consider. However the D5100 is not one of them. How much control are you going to cede to the camera? From reading several of your posts, on this thread, I don't think a whole lot. . If that is the case, you are selling yourself short if you opt for the D5100. You have enthusiasm in your photography. Because of that you may quickly out grow that camera or cameras like it. An enthusiast's (D7000, D600) or pro camera (D800, EOS 5D III) would serve you better than a basic unit. They will allow you to grow your photography without getting in your way. Cameras of such capability give you the ability to exercise your imagination, and to learn unfettered by the lack of capabilities of a beginners camera. There aren't any cameras that will teach you very much. Your learning will come from doing, supplemented by what you read.